An emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD). A constituent element of an ELD includes such as an inorganic electroluminescent element ((hereinafter, it is also referred to as an inorganic EL element) and an organic electroluminescent element (hereinafter, it is also referred to as an organic EL element). An inorganic electroluminescent element has been utilized as a flat light source, however, it requires a high voltage of alternating current to operate an emission element.
On the other hand, an organic electroluminescent element is an element provided with a constitution comprising an emitting layer containing a emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a positive hole being injected into the emitting layer to be recombined, resulting emission utilizing light release (fluorescence·phosphorescence) at the time of deactivation of said exciton; the emission is possible at a voltage of approximately a few to a few tens volts, and an organic electroluminescent element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
Further, the major feature of the organic electroluminescent elements is also in the form of a surface light source differing from conventionally employed main light sources such as a light emitting diode or a cold-cathode tube. Possible applications, which can effectively utilize the above characteristic, include light sources for lighting and backlights of various displays. In particular, it is also appropriate to employ them as a backlight of liquid crystal full color displays, of which demand is markedly increasing over recent years.
When the organic electroluminescent elements are employed as the above light source for lighting or the display backlight, they are employed as a light source which has white or so-called electric bulb color (hereinafter together referred to as white). In order to realize white light emission by employing the organic electroluminescent elements, there available are; a method which realizes white by a color mixture by preparing a plurality of light emitting materials differing in their emitted light wavelength in a single element; a method which realizes white in such a manner that light emitting pixels of multi colors, such as three colors of blue, green, and red, are separately coated, the above pixels are allowed to emit light simultaneously, and then the emitted lights are mixed; and a method which realize white employing color conversion dyes (for example, a combination of a blue light emitting material and a color conversion fluorescent dye).
However, when consideration is made based on various demands such as lower cost, higher productivity, or more convenient driving methods, which are demanded for the light source for lighting and the backlight, the method, which realizes white by a color mixture by preparing a plurality of light emitting materials differing in emitted light wavelengths in a single element, is useful for these applications, and in recent years, research and development of the same have been increasingly conducted.
The method which realizes white based on the above method will further be detailed. There are listed: a method which realize white in such a manner that two light emitting materials in a relation of complementary colors to each other, such as a blue light emitting material and a yellow light emitting material, and the colors are mixed, and a method which realizes white in such a manner that light emitting materials of three colors of blue, green, and red, are employed and the colors are mixed.
For example, it is disclosed in Japanese Patent Publication Open to Public Inspection (JP-A) No. 2004-235168 a method for preparing a white organic electroluminescent element by doping with high efficient phosphors of three colors of blue, green and red, as a light emitting material.
In recent years, in contrast to fluorescence materials, there have been actively investigated phosphorescence materials by which an organic electroluminescent element having a high luminance can be achieved (for example, refer to Patent Document 1, Non-patent Documents 1 and 2). Previously reported light emission from a fluorescence material utilizes emission from an excited singlet. Since a generation ratio of a singlet exciton to a triplet exciton is 1:3, that is, a generation probability of an emitting exciton species is 25%. On the other hand, in the case of a phosphorescence material which utilizes an emission from an excited triplet state, the upper limitation of the internal quantum efficiency will be 100% by considering the production ratio of excitons and internal conversion of a singlet exciton to a triplet exciton. The light emission efficiency will be theoretically four times at maximum of the fluorescence emitting material.
However, there has been developed no good phosphorescence emitting material which yields blue emission and has properties of high light emission efficiency, high durability and especially a prolonged driving lifetime. Therefore, it has been difficult to develop an organic electroluminescent element of practical use which employs a blue phosphorescence emitting material.
As a method to improve the lifetime for half decrease of an initial luminance, there is disclosed an example method in which the concentration of a light emitting material in a light emitting layer is changed in the thickness direction of the light emitting layer (for example, refer to Patent Document 2). However, there is no description on light emission efficiency in this document in the document. The method disclosed is not a method enabling compatibility of both lifetime and light emission efficiency.
As a method to improve both emission lifetime and light emission efficiency, there is disclosed an example method in which the same host compound is employed in the entire organic layers; and a positive hole transporting material, an electron transporting material and a light emitting material are doped with having a concentration gradient in the organic layers (for example, refer to Patent Document 3).
This method was developed based on the assumption that an interface produced by lamination of layers containing a various functional materials will lead deterioration of lifetime. When various doping materials are doped to the same host compound located in the entire organic layer, the concentration of the doping materials is controlled to be decreased to zero at an end portion of the doping area. It was expected that the interfaces will disappear by this method, and as a result, the lifetime will be prolonged. The concentrations of a light emitting material are decreased at both end portions in the depth direction of the light emitting layer and there disappear the interfaces with regards to the light emitting layer. However, the concentrations of a dope in the middle portion of the depth direction of the light emitting layer, which is attributed to be engaged in substantial light emission, remains constant. In this method, the laminated layer structure is not changed from the conventional one. Therefore, the chromaticity stability after a prolonged driving time will not be improved. Further, there is no description about the chromaticity stability.    Patent Document 1: U.S. Pat. No. 6,097,147    Patent Document 2: JP-A No. 2003-229272    Patent Document 3: JP-A No. 2005-100767    Non-Patent Document 1: M. A. Baldo et al., Nature, Volume 395, pages 151-154 (1998)    Non-Patent Document 2: M. A. Baldo et al., Nature, Volume 403, No. 17, pages 750-753 (2000)